Carboxylic Acids & Esters (OCR A Level Chemistry A): Flashcards
Exam code: H432
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- FrontCarboxylic Acids
Define carboxylic acid
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Define carboxylic acid
A carboxylic acid is an organic compound containing the carboxyl functional group, –COOH, with general formula CnH2n+1COOH. The carboxyl group is always on carbon 1, so no numbering is needed in the name.
Define ester
An ester is a carboxylic acid derivative containing the ester group, –COO–, formed by the condensation reaction between a carboxylic acid and an alcohol.
Define acyl chloride
An acyl chloride is an organic compound containing the –COCl functional group, prepared by reacting a carboxylic acid with liquid sulfur dichloride oxide (SOCl2).
Why are carboxylic acids classified as weak acids?
They only partially ionise in aqueous solution, producing low concentrations of H+ ions and carboxylate ions. The equilibrium lies to the left, so most molecules remain undissociated.
Why does esterification using an acid anhydride give a higher yield than using a carboxylic acid?
Acid anhydrides are more reactive than carboxylic acids and the reaction is not reversible, so the equilibrium does not limit the yield.
Why are acyl chlorides used as starting compounds in organic synthesis rather than carboxylic acids?
Acyl chlorides are more reactive than carboxylic acids, so reactions proceed faster and give higher yields. They are also preferred for synthesising esters from phenols, where carboxylic acids react too slowly.
Carboxylic acids with fewer than six carbon atoms are .......... in water because water molecules can form .......... bonds with the –COOH group.
Carboxylic acids with fewer than six carbon atoms are soluble in water because water molecules can form hydrogen bonds with the –COOH group.
Esterification between a carboxylic acid and an alcohol requires concentrated .......... as a catalyst and produces water as a .......... product.
Esterification between a carboxylic acid and an alcohol requires concentrated H2SO4 as a catalyst and produces water as a by-product.
Acyl chlorides undergo .......... reactions in which a small molecule adds across the C=O bond and a .......... molecule is then eliminated.
Acyl chlorides undergo addition-elimination reactions in which a small molecule adds across the C=O bond and a small molecule is then eliminated.
True or False?
Carboxylic acids react with carbonates to produce a salt, water and carbon dioxide.
True.
For example, ethanoic acid reacts with potassium carbonate to form potassium ethanoate, water and CO2 gas. The effervescence observed is a functional group test for carboxylic acids.
True or False?
Esterification of a carboxylic acid with an alcohol is a reversible reaction.
True.
The reaction is reversible and reaches equilibrium, giving only around 50% yield. Heating under reflux with a concentrated acid catalyst is used to speed up the reaction.
True or False?
Hydrolysis of an acyl chloride produces a carboxylic acid and HCl.
True.
A water molecule adds across the C=O bond and HCl is eliminated in this addition-elimination reaction, regenerating the carboxylic acid.
What products form when a carboxylic acid reacts with a metal, and what observation confirms this?
A metal salt and hydrogen gas are produced. The release of hydrogen gas causes visible effervescence.
Why does alkaline hydrolysis of an ester give a higher yield of carboxylic acid product than acid hydrolysis?
Alkaline hydrolysis is irreversible because the ester is fully hydrolysed to form a sodium carboxylate salt, driving the reaction to completion. Acid hydrolysis is reversible and only produces an equilibrium mixture.
Why does the reaction of an acyl chloride with ammonia require two moles of ammonia per mole of acyl chloride?
The first mole of ammonia acts as a nucleophile and forms the amide, while the second mole neutralises the HCl produced, forming an ammonium salt.
When a carboxylic acid reacts with an alkali, a .......... and .......... are formed in a neutralisation reaction.
When a carboxylic acid reacts with an alkali, a salt and water are formed in a neutralisation reaction.
Alkaline hydrolysis of an ester produces a .......... salt and an .........., which must then be acidified to obtain the free carboxylic acid.
Alkaline hydrolysis of an ester produces a sodium carboxylate salt and an alcohol, which must then be acidified to obtain the free carboxylic acid.
Reacting an acyl chloride with a .......... amine produces a .......... amide (N-substituted amide).
Reacting an acyl chloride with a primary amine produces a secondary amide (N-substituted amide).
True or False?
Numbers are needed in the name of a carboxylic acid to indicate the position of the –COOH group.
False.
The carboxyl group is always located on carbon 1, so no number is required in the IUPAC name.
True or False?
The ester methyl ethanoate is derived from methanoic acid and ethanol.
False.
Methyl ethanoate is derived from ethanoic acid and methanol. The first word in the ester name comes from the alcohol and the second from the carboxylic acid.
True or False?
Acyl chlorides react with phenols to form esters at room temperature without requiring a base.
False.
The reaction of an acyl chloride with a phenol requires both heat and a base, unlike the reaction with alcohols which proceeds without these conditions.
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